Titanium zeolites, i.e., synthetic molecular sieves that incorporate titanium atoms in a silicate framework, catalyze a wide variety of valuable oxidative organic reactions. The versatility of titanium zeolites, particularly TS-1, for arene hydroxylation, alkane oxidation, olefin epoxidation, thioether oxidation, Baeyer-Villiger oxidation reactions, and other important transformations is well known. For a review, see P. Kumar et al., Synleft. (1995) 289. Despite their obvious value for oxidation chemistry, titanium zeolites have apparently not been encapsulated within a polymer prior to their use to catalyze oxidation reactions.
For many titanium zeolite-catalyzed oxidations, hydrogen peroxide is the oxidant of choice. With a high active oxygen content and water as the only by-product, easy-to-use hydrogen peroxide has the potential to contribute to a “cleaner chemical industry” (see Pure Appl. Chem. 72 (2000) 1289). A key hurdle, however, is cost. Because hydrogen peroxide is relatively expensive, scientists continue to investigate ways to generate it “in situ” from molecular hydrogen and oxygen, usually in the presence of a platinum-group transition metal such as palladium. In situ-generated hydrogen peroxide has been used with titanium zeolites for propylene epoxidation and the oxidation of alkanes to alcohols and ketones (see Sci. Tech. Catal. (1994) 31) as well as benzene hydroxylation (J. Chem. Soc., Chem. Commun. (1992) 1446). It is presumably applicable to a variety of oxidation processes that utilize hydrogen peroxide. For additional examples of propylene epoxidations with titanium zeolites and in situ-generated hydrogen peroxide, see U.S. Pat. Nos. 5,973,171, 6,005,123, 6,063,942, 6,310,224, and 6,498,259.
Recently, Professor Sho Kobayashi reviewed a new kind of catalyst based on a technique called “microencapsulation” (see Chem. Commun. (2003) 449 and references cited therein; Angew. Chem., Int. Ed. 40 (2001) 3469; J. Am. Chem. Soc. 120 (1998) 2985). While polymer encapsulation has been used for years by the pharmaceutical industry to mask taste, impart storage stability, reduce stomach irritation, target delivery, or control release of drugs, benefits of the technique for catalysis are just now being realized. Kobayashi demonstrated that highly efficient catalysts can be made if the metals are enveloped within a thin polystyrene film. Microencapsulated transition metal catalysts and ways to make them are described in the Chem. Commun. article referenced above. These have been used for etherification, olefin dihydroxylation, allylic substitution, Suzuki coupling, and other organic transformations.
In sum, the value of microencapsulating transition metals for many organic reactions has been demonstrated, including at least one oxidative reaction (olefin dihydroxylation). Still unexplored, however, are oxidation reactions that use, as a catalyst, combinations of titanium zeolites and transition metals wherein at least one of these is encapsulated within a polymer.